At first glance, there is only a small difference between the two.
This question is more to do with the nature of evil itself. Is it its own entity, or does it mean every evil action?
If you believe the first of the two, you are more likely to say all evil. If you believe the second of the two, you are more likely to say all evils.
If you have never considered it, then it doesn't really matter, because fundamentally they mean the same thing!
Also, sometimes people like to catergorise evil. For example "Natural Evil" and "Moral Evil", if you think about the catergories, then want to talk about both, you would be more inclided to say all evils (because you mean both natural and moral evil).
In summary, there is no noticable difference between the two!
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